Conventional methods of manufacturing of aluminum alloy sheet for use in commercial applications such as auto panels, reinforcements, beverage containers and aerospace applications employ batch processes which include an extensive sequence of separate steps. Typically, a large ingot is cast to a thickness of up to about 30 inches and cooled to ambient temperature, and then stored for later use. When an ingot is needed for further processing, it is first “scalped” to remove surface defects. Once the surface defects have been removed, the ingot is preheated to a temperature of about 104° F. for a period of 20 to 30 hours, to ensure that the components of the alloy are properly distributed throughout the metallurgical structure. It is then cooled to a lower temperature for hot rolling. Several passes are applied to reduce the thickness of the ingot to the required range for cold rolling. An intermediate anneal or a self-anneal is typically carried out on the coil. The resulting “hot band” is then cold-rolled to the desired gauge and coiled. For non-heat-treated products, the coil is further annealed in a batch step to obtain O-temper. To produce heat-treated products, the coiled sheet is subjected to a separate heat treatment operation, typically in a continuous heat-treat line. This involves unwinding the coil, solution heat treatment at a high temperature, quenching and recoiling. The above process, from start to finish, can take several weeks for preparing the coil for sale, resulting in large inventories of work in progress and final product, in addition to scrap losses at each stage of the process.
Because of the lengthy processing time in this flow path, numerous attempts have been made to shorten it by elimination of certain steps, while maintaining the desired properties in the finished product.
For example, U.S. Pat. No. 5,655,593 describes a method of making aluminum alloy sheet where a thin strip is cast (in place of a thick ingot) which is rapidly rolled and continuously cooled for a period of less than 30 seconds to a temperature of less than 350° F. U.S. Pat. No. 5,772,802 describes a method in which the aluminum alloy cast strip is quenched, rolled, annealed at temperatures between 600° and 1200° F. for less than 120 seconds, followed by quenching, rolling and aging.
U.S. Pat. No. 5,356,495 describes a process in which the cast strip is hot-rolled, hot-coiled and held at a hot-rolled temperature for 2–120 minutes, followed by uncoiling, quenching and cold rolling at less than 300° F., followed by recoiling the sheet.
None of the above methods disclose or suggest the sequence of steps of the present invention. There continues to be a need to provide a continuous in-line method of making heat-treated (T temper) and annealed (O temper) sheet having the desired properties in a shorter period of time, with less or no inventory and less scrap losses.